U.S. patent application number 11/184527 was filed with the patent office on 2007-01-25 for apparatus and method to measure fluid resistivity.
This patent application is currently assigned to Schlumberger Technology Corporation. Invention is credited to Andrew Hieu Cao, Brian Clark, Dean M. Homan.
Application Number | 20070018659 11/184527 |
Document ID | / |
Family ID | 36745557 |
Filed Date | 2007-01-25 |
United States Patent
Application |
20070018659 |
Kind Code |
A1 |
Homan; Dean M. ; et
al. |
January 25, 2007 |
APPARATUS AND METHOD TO MEASURE FLUID RESISTIVITY
Abstract
An apparatus for measuring fluid resistivity includes a flow
line adapted to be in fluid communication with formation fluids,
wherein the flow line includes a first section comprising a first
conductive area, a second section comprising a second conductive
area, and an insulating section disposed between the first section
and the second section to prevent direct electrical communication
between the first section and the second section; a first toroid
and a second toroid surrounding the flow line around the first
section and the second section, respectively, wherein the first
toroid is configured to induce an electrical current in a fluid in
the flow line and the second toroid is configured to measure the
electrical current induced in the fluid in the flow line; and an
electronic package to control functions of the first toroid and the
second toroid.
Inventors: |
Homan; Dean M.; (Sugar Land,
TX) ; Cao; Andrew Hieu; (Houston, TX) ; Clark;
Brian; (Sugar Land, TX) |
Correspondence
Address: |
SCHLUMBERGER OILFIELD SERVICES
200 GILLINGHAM LANE
MD 200-9
SUGAR LAND
TX
77478
US
|
Assignee: |
Schlumberger Technology
Corporation
|
Family ID: |
36745557 |
Appl. No.: |
11/184527 |
Filed: |
July 19, 2005 |
Current U.S.
Class: |
324/693 ;
324/698 |
Current CPC
Class: |
E21B 49/10 20130101;
G01V 3/20 20130101 |
Class at
Publication: |
324/693 ;
324/698 |
International
Class: |
G01R 27/08 20060101
G01R027/08 |
Claims
1. An apparatus for measuring fluid resistivity, comprising: being
adapted to be in fluid communication with formation fluids, wherein
the flow line includes a first section comprising a first
conductive area, a second section comprising a second conductive
area, and an insulating section disposed between the first and the
second section to prevent direct electrical communication between
the first section and the second section, wherein the first
conductive area and the second conductive area are configured to
contact a fluid contained in the flow line such that, together with
a conductive path disposed outside the flow line, a current return
loop is formed; a first toroid and a second toroid surrounding the
flow line, wherein the first toroid is configured to induce an
electrical current in the fluid in the flow line and the second
toroid is configured to measure the electrical current induced in
the fluid in the flow line; and an electronic package to control
functions of the first toroid and the second toroid.
2. The apparatus of claim 1, wherein the first section and the
second section of the flow line each comprise a metallic tube.
3. The apparatus of claim 1, wherein the insulating section is made
of a material selected from glass, ceramic, and PEEK.
4. The apparatus of claim 1, wherein the first section and the
second section of the flow line are joined by thread engagement,
wherein an insulating coating is disposed on a thread region of the
first section, a thread region of the second section, or both
thread regions of the first section and the second section.
5. The apparatus of claim 1, wherein the first section and the
second section of the flow line are joined by heat shrink or force
fit, wherein an insulating coating is disposed on a coupling region
of the first section, on a coupling region of the second section,
or on both coupling regions of the first section and the second
section.
6. The apparatus of claim 1, wherein the first section, the second
section, and the insulating section of the flow line cornises a
continuous tube made of an insulating material, and wherein the
first conductive area of the first section and the second
conductive area of the second section each comprise a coating of a
conductive material on an inner surface of the flow line.
7. The apparatus of claim 1, further including a calibration loop
disposed alone the flow line for calibration of the first toroid
and the second toroid, wherein the calibration loop can be switched
on and off.
8. The apparatus of claim 1, wherein the first toroid and the
second toroid include calibration windings.
9. The apparatus of claim 1, wherein the apparatus is disposed in a
formation tester.
10. An apparatus for measuring fluid resistivity, comprising: a
flow line adapted to be in fluid coxmmunication with formation
fluids, wherein the flow line is made of an insulating material; a
first toroid and a second toroid surrounding the flow line and
spaced apart along the flow line, wherein the first toroid is
configured to induce an electrical current in a fluid contained in
the flow line and the second toroid is configured to measure the
electrical current induced in the fluid; a metallic housing
enclosing the first toroid, the second toroid, and a section of the
flow line, wherein the metallic housing is configured to provide a
return current path for the electrical current induced in the
fluid; and an electronic package to control functions of the first
toroid and the second toroid.
11. The apparatus of claim 10, further comprising a calibration
loop disposed along the flow line.
12. The apparatus of claim 10, wherein the first toroid and the
second toroid include calibration windings.
13. A method for measuring resistivity of a formation fluid in a
borehole, comprising: flowing the formation fluid through a flow
line of a resistivity measurement apparatus, the flow line
including a first section comprising a first conductive area, a
second section comprising a second conductive area, and an
insulating section between the first conductive section and the
second conductive section such that the first conductive area and
the second conductive area are disposed about and contact the
formation fluid flowing in the flow line and are configured such
that, together with a conductive path disposed outside the flow
line, a current return loop is formed; inducing an electrical
current in the formation fluid in the flow line using a first
toroid; and measuring the electrical current induced in the
formation fluid with a second toroid.
14. The method of claim 13, further determining a resistivity of
the formation fluid from measurement made with the second
toroid
15. The method of claim 13, further comprising calibrating the
first toroid and second toroid with a calibration loop disposed
along the flow line.
16. The method of claim 13, further comprising calibrating the
first toroid and second toroid with calibration windings included
in the first toroid and the second toroid.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Not applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF INVENTION
[0003] Boreholes are drilled into the Earth's formation to recover
deposits of hydrocarbons and other desirable materials trapped in
the formations below. Typically, a well is drilled by connecting a
drill bit to the lower end of a series of coupled sections of
tubular pipe known as a drillstring. Drilling fluids, or mud, are
pumped down through a central bore of the drillstring and exit
through ports located at the drill bit. The drilling fluids act to
lubricate and cool the drill bit, to carry cuttings back to the
surface, and to establish sufficient hydrostatic "head" to prevent
formation fluids from "blowing out" the borehole once they are
reached. When the borehole is drilled deep enough to reach a point
of interest, operations to perforate and fracture the subterranean
formation are performed to enable hydrocarbons, if present, to flow
from the formation into the newly drilled borehole. Because the
hydrostatic pressure of the column of drilling mud can be higher
than the reservoir pressures of the hydrocarbons, the hydrocarbons
may not flow from the formation into the borehole on their own.
Before full-scale recovery operations are commenced, drilling and
production operators prefer to test the formation fluids to ensure
the proper type and quantity of hydrocarbons are present in the
formation before completing the well. Once the formation fluids are
properly identified, various operations to retrieve the
hydrocarbons therein will be performed.
[0004] To test the fluids, a formation tester is typically deployed
downhole. Various formation fluid testers for wireline and
logging-while-drill applications are known in the art, including
the modular dynamic tester sold under the trade name of MDT.TM. by
Schlumberger Technology Corp. (Houston, Tex.). Detailed description
of these tools may be found in U.S. Pat. Nos. 4,860,581 and
4,936,139 issued to Zimmerman et al. and U.S. Published Patent
Application No. 2004/0104341, by Betancourt et al. These patents
and application are assigned to the assignee of the present
application and are incorporated by reference in their
entireties.
[0005] FIG. 1 illustrates a schematic of a formation tester 10
suspended in the borehole 12 from the lower end of a typical
multiconductor cable 15 that is spooled in a usual fashion on a
suitable winch (not shown) on the formation surface. The cable 15
is electrically coupled to an electrical control system 18 on the
formation surface. The tool 10 includes an elongated body 19 which
encloses the downhole portion of the tool control system 16. The
elongated body 19 also carries a selectively extendable fluid
admitting assembly 20 and a selectively extendable tool anchoring
member 21 which are respectively arranged on opposite sides of the
tool body. The fluid admitting assembly 20 is equipped for
selectively sealing off or isolating selected portions of the wall
of the borehole 12 such that pressure or fluid communication with
the adjacent earth formation 14 is established. Also included with
tool 10 are means for determining the downhole pressure and
temperature (not shown) and a fluid analysis module 25 through
which the fluid flows. The fluid may thereafter be expelled through
a port (not shown), or it may be sent to one or more fluid
collecting chambers 22 and 23, which may receive and retain the
fluids obtained from the formation. Control of the fluid admitting
assembly, the fluid analysis section, and the flow path to the
collecting chambers is maintained by the electrical control systems
16 and 18. As will be appreciated by those skilled in the art, the
electrical control systems may include one or more microprocessors,
associated memory, and other hardware and/or software to implement
the invention.
[0006] Before formation samples are collected into collecting
chambers 22 and 23, it is desirable to be certain that the fluids
are from the virgin formation, i.e., not contaminated by drilling
fluid from the invaded zone. To ensure that virgin formation fluids
are collected, a fluid analyzer 25 is used to monitor the
properties of the fluids while they are being drawn. The fluid
analysis module 25 may be an optical module, a pressure sensor
module, a resistivity module, or the like. Among these, the
resistivity module is particularly useful because of its wide
dynamic range. A typical resistivity module may include several
electrodes that are in contact with the fluid. These electrodes are
used to inject currents into the fluid and to measure the voltage
drop over a distance. An example of such a module is disclosed in
FIG. 1 (item 56 ) of U.S. Pat. No. 4,860,581, issued to Zimmerman.
FIG. 2 shows one example of such a module (sensor).
[0007] As shown in FIG. 2, the fluid resistivity is determined by a
four electrode sensor, where the four electrodes are short metal
tubes separated from each other and from the input and output flow
lines by short insulating tubes. The two outermost electrodes
inject an electrical current (I) into the fluid sample, while the
voltage drop (V) between the two innermost electrodes is measured.
With a known current (I) and the measured voltage (V), the
resistivity of the fluid is obtained.
[0008] However, these electrode devices are exposed to the fluids
in the flow line that can be relatively high pressures (up to
30,000 psi). Therefore, good seals (e.g., bulkhead, o-rings or
other mechanical seals) are necessary to protect the electronic
parts that are outside the flow line and are at atmosphere pressure
(about 14 psi). As boreholes drilled at such depths are often at
the smallest gauge diameter, such measurement equipment and the
sealing mechanisms (bulkhead and o-rings) are necessarily of a very
small form factor. In the limited volume available for the
resistivity sensor, it is difficult to achieve pressure seals
between all of the insulated tubes and metal tubes. For a sensor
shown in FIG. 2, at least eight seals would be needed; ten are
needed including the seals between the outermost insulating tubes
and the input and output fluid lines. Instead, four bulkhead
electrical feed-throughs are used for the four wires connecting the
electrodes to the electronics. At extreme temperatures and
pressures, even the four bulkhead feed-throughs can be unreliable.
As a result, there is great difficulty in producing a reliable
resistivity sensor.
[0009] Therefore, there still exists a need for methods and
apparatus for resistivity measurement that may be reliably used in
formation testers or similar downhole equipment.
SUMMARY OF INVENTION
[0010] An aspect of the invention relates to apparatus for
measuring fluid resistivity. An apparatus for measuring fluid
resistivity in accordance with one embodiment of the invention
includes a flow line adapted to be in fluid communication with
formation fluids, wherein the flow line includes a first section
comprising a first conductive area, a second section comprising a
second conductive area, and an insulating section disposed between
the first section and the second section to prevent direct
electrical communication between the first section and the second
section, wherein the first conductive area and the second
conductive area are configured to contact a fluid contained in the
flow line such that, together with a conductive path disposed
outside the flow line, a current return loop is formed; a first
toroid and a second toroid surrounding the flow line, wherein the
first toroid is configured to induce an electrical current in the
fluid in the flow line and the second toroid is configured to
measure the electrical current induced in the fluid in the flow
line; and an electronic package to control functions of the first
toroid and the second toroid.
[0011] An aspect of the invention relates to apparatus for
measuring fluid resistivity. An apparatus in accordance with one
embodiment of the invention includes a flow line adapted to be in
fluid communication with formation fluids, wherein the flow line is
made of an insulating material; a first toroid and a second toroid
surrounding the flow line and spaced apart along the flow line,
wherein the first toroid is configured to induce an electrical
current in a fluid contained in the flow line and the second toroid
is configured to measure the electrical current induced in the
fluid; a metallic housing enclosing the first toroid, the second
toroid, and a section of the flow line, wherein the metallic
housing is configured to provide a return current path for the
electrical current induced in the fluid; and an electronic package
to control functions of the first toroid and the second toroid.
[0012] Another aspect of the invention relates to methods for
measuring resistivity of a formation fluid in a borehole. A method
in accordance with one embodiment of the invention includes flowing
the formation fluid through a flow line of a resistivity
measurement apparatus, the flow line including an insulating
section between a first conductive section and a second conductive
section; inducing an electrical current in the formation fluid in
the flow line with a first toroid; and measuring the electrical
current induced in the formation fluid with a second toroid.
[0013] Other aspects and advantages of the invention will be
apparent from the following description and the appended
claims.
BRIEF DESCRIPTION OF DRAWINGS
[0014] FIG. 1 shows a schematic of a prior art formation tester
disposed in a wellbore.
[0015] FIG. 2 shows a schematic of a prior art resistivity sensor
having four electrodes.
[0016] FIG. 3 shows a schematic of a toroid resistivity sensor in
accordance with one embodiment of the invention.
[0017] FIG. 4 shows a cross-sectioned schematic view of a fluid
resistivity measurement apparatus in accordance with an embodiment
of the present invention.
[0018] FIG. 5 shows a schematic cross-sectioned view of a fluid
flow line in accordance with an embodiment of the present
invention.
[0019] FIG. 6 shows a schematic cross-sectioned view of another
fluid flow line in accordance with an embodiment of the present
invention.
[0020] FIG. 7 shows a schematic cross-sectioned view of another
fluid flow line in accordance with an embodiment of the present
invention.
[0021] FIG. 8 shows a schematic cross-sectioned view of another
fluid flow line in accordance with an embodiment of the present
invention.
[0022] FIG. 9 shows a schematic cross-sectioned view of another
fluid flow line in accordance with an embodiment of the present
invention.
[0023] FIG. 10 is a schematic cross-sectioned view of another fluid
flow line in accordance with an embodiment of the present
invention.
[0024] FIG. 11 shows a schematic that represents a resistivity
curve that might be expected when a well is drilled with a
water-based mud and when testing a zone containing oil.
[0025] FIG. 12 shows a mechanical schematic drawing of a
measurement apparatus in accordance with an embodiment of the
present invention.
[0026] FIG. 13 shows an electrical schematic drawing of a circuit
in accordance with an embodiment of the present invention.
[0027] FIG. 14 shows a resistivity measurement device including a
calibration loop or secondary windings on the toroids for
calibration in accordance with one embodiment of the invention.
DETAILED DESCRIPTION
[0028] Embodiments of the invention relate to a resistivity sensor
suitable for use in a formation tester or similar equipment. A
resistivity sensor in accordance with embodiments of the invention
does not rely on electrodes that are in direct contact with the
fluids for resistivity measurement. Instead, a resistivity sensor
in accordance with embodiments of the invention uses toroids to
inductively measure the resistivity of the fluids in a flow line. A
resistivity sensor in accordance with embodiments of the invention
may be used in a fluid analyzer of a formation tester (e.g., 25 in
FIG. 1).
[0029] FIG. 3 shows a schematic of a toroid-based resistivity
sensor in accordance with one embodiment of the invention. As shown
in FIG. 3, the resistivity sensor 30 includes a flow line 35 that
is comprised of an insulating segment 33 flanked by two conductive
segments 31, 32. Note that the insulating segment 33 is shown to be
made entirely of an electrically insulating material (e.g.,
ceramic, glass, PEEK, etc.), while the conductive segments 31, 32
are entirely made of a conductive material (e.g., metal). However,
one of ordinary skill in the art would appreciate that the
insulating segment 33 may also comprise a conductive body with an
insulating coating on the inner surface provided there is not a
continuously conductive path along its length that allows current
to flow between conductive segments 31 and 32. Similarly, the
conductive segments 31, 32 may also be made of a non-conductive
material with a conductive coating on the inner surface. Various
modifications of the construction of a flow line in accordance with
embodiments of the invention will be illustrated with several
examples in a later section, with reference to FIGS. 5-10.
[0030] Referring again to FIG. 3, two toroids T1, T2 are disposed
around (or circumscribing) the conductive segments 31, 32. One of
ordinary skill in the art would know that a toroid comprises a
donut-shaped core, which is typically made of a ferrite or other
ferromagnetic material, and conductive wire windings on the core.
When a current is passed through the conductive wire winding, a
magnetic field is induced. The induced magnetic field, which is
substantially aligned with the circular path of the core, can
induce a current in a conductive material surrounded by the core.
The induced current flows in a direction parallel with the axis of
the core.
[0031] Thus, when toroid T1 is energized, it inductively induces a
current in the fluids in the flow line 35. The presence of an
insulating segment 33 prevents the induced electric current from
lowing directly from the first conductive segment 31 to the second
conductive segment 32 and forces the induced current to flow
through the fluid column contained in the insulating segment 33
(shown as dotted arrows 39) to reach the second conductive segment
32. A current return path 38 then allows the current to return to
the conductive segment 31 and completes the circuit. The current
induced in the second conductive segment 32 and the fluid therein
in turn inductively induces a current (or voltage) in the second
toroid T2. The current or voltage detected in toroid T2 may be
compared with the current (or voltage) applied to toroid T1 to
calculate a resistance of the fluid (R.sub.f) across the insulating
segment 33. This resistance (R.sub.f) is a function of the
resistivity of the fluid (.rho..sub.f), the cross section area (A)
of the flow line 35, and the length (l) of the insulating segment
33. That is, R.sub.f=K.times..rho..sub.f.times.l/A, where K is a
constant that depends on the geometry and has a value close to 1. K
can be determined from a one-time calibration using a fluid with a
known resistivity. Accordingly, the resistance (R.sub.f) can be
determined from the measured voltage. Hence, the resistivity of the
fluid (.rho..sub.f) may be obtained from the known length (l) of
the insulating segment 33, and known cross section (A) area of the
flow line 35, and the known factor K. Details of such calculation
will be described later with reference to FIGS. 12 and 13.
[0032] A resistivity sensor (or apparatus) as illustrated in FIG. 3
may be incorporated into various downhole tools to measure fluid
resistivity in a flow line. One example, as illustrated in the
following description, is to incorporate such a resistivity sensor
in a fluid analysis module (shown as 25 in FIG. 1) of a formation
tester (e.g., MDT.TM.).
[0033] FIG. 4 shows a fluid resistivity measurement apparatus 100
in accordance with one embodiment of the invention. Resistivity
measurement apparatus 100 is preferably constructed such that the
resistivity of fluids flowing through a flow line 102 can be
inductively measured. Fluid flow line 102 shown includes an inlet
section 104 and an exit section 106 (corresponding to the
conductive segments in FIG. 3) separated by an insulating section
108. Insulating section 108 prevents electric currents from
traveling directly from inlet section 104 to exit section 106.
Inlet and exit sections 104, 106 are preferably constructed of a
high-strength material, such as metal, PEEK, ceramic, etc. As noted
above, if these segments are made of non-conductive materials
(e.g., PEEK or ceramic), the inner surface of the sections may be
coated with a conductive material to provide electrical contacts
with the fluid. These electrical contacts form part of the loop
(shown as 38 in FIG. 3) that provides current return. Note that if
the fluid input and output lines are made of metal, then the fluid
input and output lines can provide electrical contacts with the
fluid.
[0034] The resistivity measurement apparatus 100 also includes a
pair of toroids 110 and 112 surrounding the flow line 102. The
toroids 110 and 112 are separated axially by a spacing gap 114. End
caps 116, 118 retain toroids 110, 112 within resistivity
measurement apparatus 100. First toroid 112 may act to induce a
current in the fluid flowing through the flow line 102 and second
toroid 110 may detect that current (or induced voltage), or vice
versa. Because toroids 110, 112 measure currents in the fluids
indirectly without having to contact the fluid, they can perform
their functions from outside the high pressure flow line.
Resistivity measurement apparatus 100 includes an electronics
package 120 to drive toroid 110 or 112 and make resistivity
measurements or calculations. Because electronics package 120,
toroids 110, 112, and all wires and leads therebetween are not
exposed to the elevated pressures in the flow line 102, complex
sealing and hydraulic isolation mechanisms are not necessary.
Therefore, resistivity measurement apparatus 100 will be much more
reliable in operation in that the risk of catastrophic failure in a
hydraulic seal protecting electronics and sensors is minimized.
[0035] In this particular example, all components of resistivity
measurement apparatus 100 are encased within a housing 122 that is
adapted to fit within a measurement and testing device (e.g., an
MDT.TM.) for delivery to a downhole location. For such
applications, the resistivity measurement apparatus 100 is
preferably dimensioned to fit in an existing tool. In accordance
with one embodiment of the invention, housing 122, for example, may
be a 2.0-inch diameter cylindrical housing to fit in an MDT.TM.
tool. A measurement and testing device of this size necessarily
means that the diameter of the flow line 102 is small (e.g., less
than 0.250'' inches (0.6 cm)) in diameter. If a conventional
electrode device were used, hydraulic seals needed to seal around
the electrodes will necessarily be of very small dimensions and
will likely have difficulty withstanding pressures exceeding 30,000
PSI and temperatures up to 200.degree. C. By using toroids, it is
possible to sequester all electronics from the elevated pressures
and, therefore, no hydraulic seals are necessary.
[0036] As noted above, various configurations are possible for
constructing a flow line having an insulating segment disposed
between the two conductive segments. In one embodiment, a flow line
may comprise an insulating section 108 made of glass (or other
insulating materials, such as ceramic or PEEK
(polyetheretherketone)) joined at both ends thereof by conductive
sections (e.g., sections 104 and 106 in FIG. 4) that are made of
metal. Glass to metal joints have been successfully used in
downhole tools. Glass can provide the insulation and at the same
time can withstand relatively high pressure. Other materials
including, but not limited to, high-temperature plastics (e.g.,
PEEK) and ceramics may also be used.
[0037] Referring to FIG. 5, a flow line 102 assembly similar to
that of FIG. 4 is shown schematically. A glass to metal seal is
formed between sections 104 and 106 such that the glass portion of
the seal acts as the insulating section 108, whereby an insulated
gap 130 exists between sections 104 and 106. When induced by toroid
112, electrical current (shown schematically by lines 132) flows
between section 104 and section 106 in the fluid through the flow
line 102. In the example shown in FIG. 5, the length of current
flow 132 is approximately the same as the size of axial gap between
sections 104 and 106. However, because glass portion 108 is of
significantly less tensile and shear strength than metal section
sections 104 and 106, the glass span is relatively more susceptible
to radial stresses and breakage when highly pressurized fluids pass
through flow line 102. This will also be true with a plastic
insulating section. Therefore, it is desirable to reduce the axial
span of the glass (or plastic) section.
[0038] Referring now to FIG. 6, another flow line 402 is shown
schematically. Flow line 402 includes conductive sections 404 and
406 that are axially closer together than that of FIG. 4. With less
of an axial gap between sections 404 and 406, the radial stresses
in the insulating section 408 is dramatically reduced. However, if
the axial gap is reduced, the resistivity measurement may not be
accurate because the measured resistance is a function of the
insulating segment (l). Therefore, to compensate for the reduced
axial span, the insulating section 408 may be extended along a
length 430 on the inner surface of the flow line 402. The portion
of insulating section 408 inside the bore of flow line 402 can be
relatively thin, for example a light coating. Current flow 432
travels substantially the same distance as current flow 132 of FIG.
5, but the span of weak glass (or other material) within insulating
section 408 is significantly reduced. Therefore, the insulating
section 408 is much less susceptible to failure under heavy
pressure loads than the insulating section 108 of FIG. 5.
[0039] FIG. 7 shows a flow line 502 in accordance with another
embodiment of the invention. In FIG. 7, the flow line 502 is
primarily comprised of a first section 504 and a second section 506
joining together with a thread engagement, wherein an insulating
coating exists on threads 534. The first section 504 and the second
section 506 may be made of metal. The insulating coating on threads
534 effectively prevents electrical communication between section
sections 504 and 506 and requires current 532 to flow through the
fluid in the flow line 502. In order to extend the axial length
that current 532 must travel, an insulating section 508 of length
530 is coated on the inner surface of the flow line 502. Insulating
section 508 can be of any insulating material, such as glass,
ceramic, rubber, PEEK, etc. Because insulating section 508 is
backed by the metal sections 504 and 506, it should be able to
withstand substantial radial stress from the high-pressure fluid in
the flow line 502. The possibility of failure is minimized.
[0040] One of ordinary skill in the art would appreciate that
variations of the embodiment shown in FIG. 7 are possible. For
example, FIG. 8 shows an embodiment that has a thread connection,
wherein an insulating material 834 (e.g., ceramic) is coated on
either or both of the thread ends to prevent electrical
communication between the fluid inlet 804 and outlet 806 lines. The
ceramic coating is typically 0.010 to 0.020 inches (0.25-0.49 cm)
thick. Ceramic coatings are available from vendors such as Praxair,
Inc. The mechanical load from the internal pressure is carried by
the metal tubes so that this design is very robust. The insulating
region between the two metal tubes is obtained by adding an
insulating layer 831, which can be a ceramic coating, a rubber
layer molded onto the metal tubes, or an insert of plastic, glass,
or PEEK. The insulating layer 831 having a length 830 forces the
current to travel via the fluid column in this section.
[0041] FIG. 9 shows a similar embodiment, but with a non-threaded
coupling. As shown in FIG. 9, the fluid inlet 904 and outlet 906
lines are coupled by a shrink-fit mechanism. In assembling the two
tubes, the outer metal tube is heated so as to expand it, which
allows it to be slid over the inner tube. When the outer tube cools
to the same temperature as the inner tube, the two tubes are
compressed and form a pressure seal. The coupling region (on the
inner tube, outer tube, or both) is coated with an insulating
material 934, such as ceramic (typically with a thickness of
0.010-0.020 inches (0.25-0.49 cm), to prevent direct electrical
communication between the inlet 904 and outlet 906 lines. An
insulating layer 931 is coated on the inside of the tube to provide
the insulating segment. Another possibility is to use O-ring seals
in the coupling region rather than a heat shrink fit to provide a
pressure barrier (not shown).
[0042] FIG. 10 shows a flow line 702 in accordance with another
embodiment of the invention. As shown in FIG. 10, the flow line 702
is mainly comprised of an insulating tube 703 that includes two
sections of conductive coatings 704, 706. The conductive coating
sections 704, 706 are separated by an uncoated section 730. The
conductive coatings 704, 706 are connected externally by a wire or
by the metal housing surrounding the sensor. The current 732 flows
between the two conductive coatings.
[0043] FIGS. 5-10 illustrate several flow lines in accordance with
embodiments of the invention. These examples are for illustration
only. One of ordinary skill in the art would appreciate that other
modifications are possible without departing from the scope of the
invention. For example, FIG. 12 shows a resistivity sensor in
accordance with another embodiment of the invention, comprising a
flow line 602 made of a non-conductive material (e.g., PEEK or
ceramic) and a metal housing 640 that encloses the two toroids 610,
612. In this embodiment, the metal housing provides current return
paths at points where the flow line 602 exits the metal housing
640. At these locations, the fluid comes in contact with metal
fluid input and output flow lines.
[0044] A resistivity measurement apparatus in accordance with
embodiments of the invention may be used to monitor resistivity
changes, for example, while the formation fluids are being drawn in
to a formation tester. When the resistivity reaches a steady state,
it may be assume that the fluids being drawn into the formation
tester are representative of virgin formation fluids, i.e.,
substantially free of the invaded drilling fluid. FIG. 11 shows a
schematic that represents a resistivity curve that might be
expected when a well is drilled with a water-based mud and when
testing a zone containing oil. As shown, the initial resistivity of
the fluids is highly influenced by the conductive drilling fluids
that have invaded the formation. As more fluids are drawn into the
formation tester, the proportion of the invaded drilling fluids
decreases, while the proportion of the resistive virgin formation
fluid increases. Eventually, the resistivity detected in the flow
line is expected to approach that of the virgin formation fluids,
i.e., approaching a steady state. Thus, a resistivity sensor of the
invention may be used to monitor when the fluids drawn into a
formation tester are representative of virgin formation fluids and,
therefore, are suitable to be collected for later analysis.
[0045] In addition to "qualitative" applications described above
with reference to FIG. 11, a resistivity measurement apparatus in
accordance with embodiments of the invention may also be used to
determine the resistivities of the fluids (quantitative
applications).
[0046] Referring now to FIGS. 12 and 13, the physics behind the
indirect measurement of fluid resistivity with two toroids will be
described. FIG. 12 depicts a fluid resistivity cell 600 having two
toroids 610, 612 to measure resistivity of fluids in a flow line
602 by inductive coupling. As described above in reference to FIG.
4, the first toroid 612 induces a current in the flow line 602 and
the second toroid 610 measures the induced current. The flow line
602 shown in FIG. 12 is constructed of an electrically
non-conductive tube having an internal radius a and a length l. The
fluid resistivity cell 600 in FIG. 12 is encased within a
cylindrical metallic housing 640 and includes a internal cavity 642
filled with a nonconductive material having a nominal magnetic
permeability .mu..sub.0, including, but not limited to atmospheric
air, vacuum, or an insulating polymeric material (e.g. epoxy,
rubber, fiber glass, plastic, PTFE or PEEK).
[0047] The magnitude of the current induced in flow line 602
depends on the resistivity of the fluid (.rho..sub.f) flowing
through the electrically non-conductive line 602 and various
parameters of toroids 610, 612. Each toroid 610, 612 has a inner
radius b, an outer radius c, a thickness h, a number of turns of
wire wrapped thereupon N, and a permeability .mu.'. Preferably,
toroids 610, 612 have a high magnetic permeability .mu.' and can be
manufactured of ferrite, iron powder, mu-metal, superalloy, or any
other material appropriate for the operating frequency. The
operating frequency may be any frequency that can induce a current
in the fluids in the flow line, for example, in a range of 5 KHz to
200 MHz, preferably 20 KHz to 10 MHz, more preferably 20 KHz to 2
MHz. Furthermore, each toroid 610, 612 may include an electrostatic
shield to eliminate/minimize any capacitive coupling or direct
mutual inductive coupling between the toroids 610, 612.
[0048] The self inductance of toroids 610, 612, as shown in FIG.
12, can be characterized by L = .mu. 0 .times. .mu. ' .times. N 2
.times. h .times. .times. ln .function. ( c / b ) 2 .times. .pi. (
Eq . .times. 1 ) ##EQU1## where .mu..sub.0=4.pi.10.sup.-7 Henry/m.
The mutual inductance between each toroid 610, 612 and the fluid in
the flow line 602 can be described as: M = .mu. 0 .times. .mu. '
.times. Nh .times. .times. ln .function. ( c / b ) 2 .times. .pi. .
( Eq . .times. 2 ) ##EQU2## The self-inductance of one half of the
fluid-filled flow line 602 is L f = .mu. 0 .times. .mu. ' .times. h
.times. .times. ln .function. ( c / b ) 2 .times. .pi. + .mu. 0
.times. h 2 .times. .pi. .function. [ ln .function. ( bd ac ) + 1 4
] + .mu. 0 2 .times. .pi. .times. ( / 2 - h ) .function. [ ln
.function. ( d a ) + 1 4 ] ( Eq . .times. 3 ) ##EQU3## where the
first term dominates because .mu.'>>1. Note that d is the
inner radius of the metallic housing 640. As noted above, the
resistance of the fluids in flow line 602 is a function of the
resistivity of the fluid (.rho..sub.f), the length of the fluid
path (l), and the cross-sectional area of the tube (A=.pi.a.sup.2):
R f = K .times. .rho. f .times. .pi. .times. .times. a 2 . ( Eq .
.times. 4 ) ##EQU4##
[0049] Referring to FIG. 13, a known current I.sub.1 (from a source
with a voltage V.sub.1) energizes first toroid 612, thereby
inducing a current I' in fluid line 602, which returns through the
metal end faces and the metal housing (see FIG. 12). This induced
current I' produces a current I.sub.2 (or voltage V.sub.2) in the
second toroid 610. The output of the second toroid 610 is attached
to an operational amplifier (not shown), preferably with high input
impedance.
[0050] The circuit model shown in FIG. 13 can be used to illustrate
the processes involved in solving for R.sub.fand, hence,
.rho..sub.f. For the first toroid 612,
V.sub.1=j.omega.LI.sub.1-j.omega.M I' (Eq. 5)
V''=-j.omega.L.sub.fI'+j.omega.M I.sub.1. (Eq. 6) For the second
toroid 612, V'=j.omega.L.sub.fI'+jM I.sub.2 (Eq. 7)
V.sub.2=j.omega.LI.sub.2+j.omega.M I'. (Eq. 8) For fluid line 602,
V''=V'+I' R.sub.f. (Eq. 9) Solving for V.sub.2 yields, V 2 =
j.omega. .function. [ L - M 2 2 .times. L f - j .function. ( R f /
.omega. ) ] .times. I 2 + j.omega. .function. [ M 2 2 .times. L f -
j .function. ( R f / .omega. ) ] .times. I 1 ( Eq . .times. 10 )
##EQU5## where both terms have loss and reactance. If V.sub.2 is
measured with a sufficiently high impedance operational amplifier,
then it can be assumed that I.sub.2 is zero, and the above equation
reduces to V 2 = - .omega. 2 .function. ( M 2 R f + j2.omega.
.times. .times. L f ) .times. I 1 . ( Eq . .times. 11 )
##EQU6##
[0051] Substituting the self-inductance into the above equation
gives the relationship between the measured quantities for V.sub.2
and I.sub.1 and the desired quantity R.sub.fas follows: V 2 I 1 = -
.omega. 2 .times. .mu. 0 2 .function. ( .mu. ' ) 2 .times. N 2
.times. h 2 .times. ln 2 .function. ( c / b ) 4 .times. .times.
.pi. 2 .times. ( 1 R f + j2.omega. .times. .times. L f ) . ( Eq .
.times. 12 ##EQU7## This equation can now be inverted and combined
with Equation 4 to find R.sub.fand .rho..sub.f. It should be noted
that the relative magnetic permeability (.mu.') is a squared term
in the numerator of Equation 12, and it also appears in the
denominator as a component of L.sub.f(see Equation 3).
[0052] The relative magnetic permeability .mu.' of toroids 610, 612
may be temperature-dependant and, therefore, may need to be
calibrated over the operating temperature range. If the temperature
variation of .mu.' is small and predictable, then a
temperature-dependent correction can be applied to the reading, if
necessary. One approach is to measure the permeability .mu.' of the
toroids at various temperatures prior to deployment downhole. Once
in position, a sensor in the fluid resistivity cell 600 may measure
the temperature and insert a correction factor from a look-up table
into the calculation.
[0053] If the temperature variation of .mu.' is large or
unpredictable, it may be necessary to include a calibration
function into the system. One approach is to add secondary windings
(S1, S2 in FIG. 14) to each toroid 610, 612, so that these toroids
may be calibrated by injecting a known current and by measuring the
voltage induced thereby. Secondary windings (e.g., S1) can also be
used to monitor applied voltage (e.g., V.sub.1). Another approach
would be to pass a conductor (e.g., a conductor wire or calibration
loop (CL in FIG. 14)) through both toroids 610, 612 parallel to
fluid line 602. The calibration loop (CL) is in series with a known
resistance R.sub.c and a switch to open and close the circuit. The
voltage V.sub.2 could be measured with the calibration loop both
open and closed. In this case, the open loop measurement is:
X=-k.mu.'.sup.2(G) (Eq. 13) and the closed loop measurement is: Y =
k .times. .times. .mu. '2 .function. ( G + S ) , .times. where ( Eq
. .times. 14 ) G = 1 R f + j2.omega. .times. .times. L f , .times.
and ( Eq . .times. 15 ) S = 1 R c ( Eq . .times. 16 ) ##EQU8## X
and Y are measured quantities, S is known, and G and .mu.' are
unknowns. Solving the above Equations 13-16 results in G = S ( Y /
X - 1 ) . ##EQU9##
[0054] Note the overall system (see FIG. 13) resistance depends not
only on the fluid resistance (R.sub.f) in the flow line, but also
on the efficiency of the inductive coupling between the fluid and
the toroids. Inductive coupling efficiency depends on the frequency
(.omega.) and the inductance of the fluid (L.sub.f). In order to
have reliable measurement of the fluid resistivity (.rho..sub.f),
it is preferable that the system design and the operating frequency
are chosen so that R.sub.f.gtoreq..omega.L.sub.f. In designing a
system, it is important to keep in mind that .rho..sub.fand
R.sub.fmay vary over many orders of magnitude. This point is
particularly important when measuring low resistivity fluids, such
as water-based mud or formation water.
[0055] In practice, the toroidal fluid resistivity cell could be
operated with an ideal voltage drive for V1 and an ideal current
measurement of I2, in which case we can write the expression V
.times. .times. 1 I .times. .times. 2 .times. C = R f , .times.
where ##EQU10## C = V .times. .times. 1 I .times. .times. 2 .times.
1 R cal . ##EQU10.2## Here C is the calibration factor found by
measuring the response of a wire loop run through the two toroid
with a known resistance R.sub.cal. The calibration resistor is
assumed to be a precision resistor with a temperature stability of
5 ppm/C.
[0056] The above description uses a formation tester to illustrate
embodiments of the invention. One of ordinary skill in the art
would appreciate that embodiments of the invention may also be used
in other applications, such as open hole wireline tools, cased hole
wireline tools (e.g., cased hole dynamic tester, CHDT.TM., a
trademark of Schlumberger Technology Corp. (Houston, Tex.)),
logging-while-drilling tools, permanent monitoring (in both
downhole and surface equipment), and in other flow lines
(particularly, those subjected to high pressures).
[0057] While the invention has been described with respect to a
limited number of embodiments, those skilled in the art, having
benefit of this disclosure, will appreciate that other embodiments
can be devised which do not depart from the scope of the invention
as disclosed herein. Accordingly, the scope of the invention should
be limited only by the attached claims.
* * * * *